1. Field of the Invention
This invention relates to an image forming apparatus with edge smoothing.
2. Description of the Prior Art
Image forming apparatus for forming a video image from a continuous tone video signal through a half tone processing are known. A laser beam printer as one of such image forming apparatus provides a half tone image by scanning a photosensitive material with a laser beam which is pulse-width-modulated by a video density signal. A color laser printer forms a color image by combining halftone images of primary colors and reproduces a color image on a recording paper. A black halftone image may be added to the combined image.
FIG. 15 is a block diagram of a prior art laser beam printer. A d/a converter 1 receives video density data and latches the video density data in response to a clock signal which is in phase with the video data and converts video data into an analog video density signal. A reference signal generation circuit 2 generates a reference signal having sawtooth waves for pulse width modulation in response to a clock signal. A comparing circuit 3 compares the analog video density signal with the reference signal to produce a pulse-width-modulated signal. A laser diode 4 generates a pulse-width-modulated laser beam scanned on a photosensitive member (not shown). The number of tones reproduced depends on the number of bits of the video density data. For example, if the video density data has eight bits data, tones of 256 steps can be reproduced.
FIG. 16 shows waveforms respective signals of this prior art image forming apparatus.
The reference signal generation circuit 2 generates the reference signal having sawtooth waves in response to the clock signal as shown by waveforms 101 and 102. The comparing circuit 3 compares the video density signal as shown by waveform 103 with the reference signal as shown by waveform 102 and outputs the pulse-width-modulated signal such that a logic high level is outputted when the magnitude of the video density signal is larger than that of the reference signal and a logic low level is outputted when the magnitude of the video density signal is smaller than that of the reference signal as shown by wave form 104. One pixel corresponds to one period of the clock signal as shown by waveform 101 and to an interval between two consecutive peaks of the reference signal. A valley of the reference signal corresponds a center of the pixel. The laser diode 4 produces the laser beam pulse-modulated by the video density signal and forms a halftone image on the photosensitive member through scanning. When video density data indicative of the most shadow level, the pulse-width-modulated signal shows the logic high level during 100% of the one pixel period, that is, a maximum dot is formed. When video density data is indicative of a tone of 50% of the maximum dot, the pulse-width-modulated signal shows the logic high level during 50% of the one pixel period. That is, the pulse-width-modulation signal is produced such that a dot formed in a pixel grows around the center of the pixel in accordance with the magnitude of the video density signal when the dot is seen on the printed copy.
FIG. 17 is an illustration of a video image reproduced by the prior art image forming apparatus mentioned above. FIG. 18 shows waveforms observed at respective portions of the prior art image forming apparatus when the video image shown in FIG. 17 is formed. As shown in FIG. 17, at the second pixel (dot) on the second line, the video density data indicates 100% dot, the laser diode 4 emits the laser beam during one pixel period because the video density data is larger in magnitude than the reference signal during the pixel period. At other pixels on the second line, no dot is formed because the video density data is smaller in magnitude than the reference signal during the pixel periods of these pixels.
When video density data such that the video image is reproduced as shown in FIG. 17 is inputted to the prior art image forming apparatus shown in FIG. 15, the reproduced video image provides a user unpleasant feeling because there is an edge serration between the second and third lines.